Why shift current doesn't depends on applied voltage?

Question

I am currently reading about the Bulk photovoltaic effect in Weyl semi-metal. Shift current is one of the major sources of current in the Bulk Photovoltaic effect. They originate in non-centrosymmetric materials. They don't need an applied field for charge separation when illuminated by a light source, whereas conventional p-n junction solar cell requires a field.

It is also mentioned that (ref [paragraph above eq.10][1]  ) the shift current doesn't depend on the external voltage.

In my understanding since the non-centrosymmetric materials have a 'direction'([refer intro. second paragraph][2]), and photoexcited electrons will move along the polarisation(direction) of the material

My question is, If the process is as above-mentioned, does the photocurrent density actually depend on the external voltage? Because the current density is the rate of change of charge per unit time per unit area; which will increase as we increase the external voltage.

What am I missing. Can someone help me with this!